1. Field of the Invention
The present invention relates in general to method and apparatus for distance determination, and relates in particular to a distance measuring technology applicable to various kinds of manufacturing machines and evaluation apparatuses.
This application is based on patent application No. Hei 9-335889 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
Distance measuring apparatus is an integrated part of various equipment for manufacturing, fabricating, measuring and evaluating activities, and non-contacting distance measuring apparatus (distance sensor) is a known example in such applications.
Non-contacting distance sensors include ultrasonic and laser range sensors, but laser range sensors are preferred when the application requires rapid response and high precision.
FIG. 16 shows a schematic illustration of how a conventional laser range sensor operates.
In this diagram, 1 represents a light source, 2 a light beam generated from the light source 1, 3 a measuring object, 7 a reflected beam from the surface of the measuring object 3, 8 an optical member, 9 a photo-detector member 9, and 10 a light receiving surface of the member 9.
As shown in this diagram, laser range sensor is comprised by a laser beam output section for emitting a laser beam 2 generated from the light source 3 towards the measuring object 3, and a light input section for focusing the reflected beam 7 leaving the surface of the measuring object 3 at a light spot on the surface of the light receiving surface 10 of the photo-detector 9 through an optical member 8.
Photo-detector 9 is a member to convert the luminous energy falling on the light spot focused on the light receiving surface 10 to electrical signals, and may include, for example, a one-dimensional charge coupled device (CCD) or one-dimensional position sensitive device (PSD).
The optical member 8 may commonly be a mirror, prism, or lens, but in FIG. 16, the laser range sensor uses only a lens for the optical member.
Also, although not shown in the diagram, a laser range sensor is generally provided with a control section for controlling the output and input sections and a computation section for determining the distance to the object according to measured data from the input section.
Such laser range sensors operate by directing a light beam 2 to form a light spot on the surface of the measuring object 3 generated from the light source 1, and forming another light spot on the receiving surface 10 created by focusing a reflected beam 7 through the optical member (receiving lens) 8.
In this case, as illustrated in FIG. 17, the focal position on the receiving surface 10 changes depending on the distance to the measuring object 3.
The distance to the object 3 can be determined by calibrating the correlation between the focal positions and distances, in terms of the known distances to the objects (3a, 3b).
Further details of range sensors are discussed in references, such as "Use and Problems of Optical Devices" (Sueda Tetsuo, Optronics, 1995).
One of the critical parameters in determining the precision of measurement by laser range sensor is the precision by which the positions of the light spots on the light receiving surface 10 of the photo-detector 9 are determined.
When the photo-detector 9 is made of a one-dimensional CCD, the light receiving surface 10 is comprised by a number of pixels disposed along a straight line, and the luminous energy of the light falling on each pixel can be converted to electrical signals of given magnitudes.
Therefore, by processing the electrical signals and obtaining a peak position or a weighted average position of luminous energy, it is possible to know which pixel position corresponds to the focal position of the reflection light spot.
When the photo-detector 9 is made of a one-dimensional PSD, it is possible to know the focal position of reflection light spot by processing the electrical signals output from the PSD to give the weighted average position of luminous energy as a ratio to the total length of the PSD, as illustrated in FIG. 18.
Accordingly, in principle, laser range sensors determine the distance by receiving a beam reflected only once from m the object 3 (simple reflection beam 7) in the photo-detector 9.
When the surface of the object 3 is glossy, a beam first reflected from the object surface may be reflected again by other surfaces (causing multiple reflections) and then return to the sensor, such that there for cases in which the simple reflection beam 7 becomes mixed with multiple reflection beam and the correct position of the simple reflection beam 7 cannot be determined with precision. In such a case, the measurement precision is significantly reduced.
To understand the loss of measurement precision in more detail, it is necessary to explain how multiple reflections affect the precision of measurements.
When multiple reflections occur, a plurality of light points are produced on the light receiving surface for that the method based on peak luminosity cannot provide the correct position of the simple reflection beam 7, because the position corresponding to the peak luminosity does not necessarily indicate the position of the simple reflection beam 7.
Using the method of weighted average luminosity, weighting tends to be shifted towards the positions of multiple reflection, and again it is not possible to determine the correct position of the simple reflection beam 7. This effect is illustrated in FIG. 19.
Multiple reflection is classified as either 2nd-order (reflected twice), 3rd-order (three times) or 4th-order multiple refection, depending on the number of times the light is reflected during the interval from leaving the light source 1 to entering into the photo-detector 9.
An example of a multiple reflection is illustrated in FIG. 20 using a case involving a 2nd-order reflection beam 15.
The 2nd-order reflection beam 15 can be avoided to some extent by orienting and operating the photo-detector 9 properly. This will be explained below with reference to FIGS. 21 and 22.
When the detector 9 and a scanner mirror 12 are oriented as shown in FIG. 21, the 2nd-order reflection beam 15 is received in the detector 9, but when they are oriented as shown in FIG. 22 to coincide the two beams, the 2nd-order reflection beam 15 cannot be received by the detector 9.
Therefore, by operating the measuring equipment suitably by redirecting the incident beam with respect to the object 3 with the use of a scanner mirror 12, it is possible to avoid detrimental effects of the 2nd-order reflection beam 15.
A 3rd-order reflection beam 16 such as the one illustrated in FIG. 23 is generated when the surface of an object 18 is glossy and another object 19 (for example, without glossy surface) is present nearby.
As illustrated in FIG. 23, a 3rd-order reflection 16 is produced when a light beam 2 emitted from a light source 1 is specularly reflected (as from a mirror surface) from the surface of a glossy object 18, which is the measuring object 3, and is then reflected diffusely from the surface of a dull object 19, and the diffused reflected light is reflected, for the third time, from the surface of the measuring object 3 before reaching the detector 9.
In other words, this pattern of reflection may be said to be a result of the surface of the glossy object 18 acting as a mirror to generate a mirror image 20 of the dull object 19 so that a light spot focused by the 3rd-order reflection represents the distance to the mirror image 20.
It should be noted that the 3rd-order reflection beam 16 cannot be avoided by simply altering the arrangement of components in the optical system, because the laser range sensor, which is a light-based system, follows the basic principles of optics.
Although multiple reflection beams of 4th-order or higher do exist in principle, optical power is attenuated at each reflection so that adverse effects of reflection beams of higher than 4th-order can be neglected in practice.